1. Field of the Invention
The present invention relates to a length measuring system comprising a scale member and a scanning unit displaceable along the scale member, the scale member and scanning unit being encased in a protective carrier housing, the scale member being connected to the housing in a manner permitting a relative longitudinal displacement, for example by an elastic connection, and fastening devices affixing the center and end parts of the carrier housing to an object, such as a machine bed. Differences in the thermal expansion of the object and carrier housing are compensated by length equalizing elements.
2. Description of the Prior Art
Published European patent application No. 110,059 discloses such a length measuring system. The disclosed structure is designed to prevent a displacement or to permit only a negligible displacement of the reference point on the scale member, which is usually at the center thereof, relative to the associated reference point of the object to which the carrier housing is affixed, for example the measuring zero point of a machine. This system uses the connection of the scale member, which is usually made of glass, to the carrier housing, which may be made of aluminum, by means of an adhesive of high elasticity, as disclosed in German patent No. 2,643,304, the scale member being disposed flat on a thin adhesive coating and the center of the scale member being fixed by an adhesive of less elasticity. To compensate for differences in the thermal expansion of the object and the carrier housing, fastening devices incorporating length equalizing elements according to German patent No. 2,853,771 are used to affix the ends of the carrier housing to the object. This permits a longitudinal displacement of the carrier housing ends relative to the fastening points of the fastening devices. The length equalizing elements in the fastening devices may take various forms. The displaceable construction produces an expensive and somewhat unstable structure of the end fastening devices, and sometimes substantially increases the length of the measuring system because of the space required for the length equalizing elements in the fastening devices. It also produces an unfavorable movement when the temperature rises, which usually occurs faster than cooling, the increase in temperature causing the parts of the carrier housing which expand more strongly than the object to move apart from the center zero point. If the machine support is uneven and the end fastening devices for the carrier housing are not exactly aligned with the center fastening device, resultant torsions in the fastening devices may interfere with the functioning of the length equalizing elements.